Wednesday, September 05, 2007

Baffled About Baffling

The Mooney is a fast airplane because every component was carefully optimized for speed and efficiency. The engine cowling, like most airplanes, is the one area that generates more drag than any other part of the airframe. The Kerrville engineers, which was led by Roy LoPresti at the time, did everything they could to design the 252 cowl with a low drag coefficient while continuing to supply ample cooling air to the engine.

The LoPresti Team did a great job with the 252. It is well known for being one of the coolest running turbos ever. However, it is critical that ram air flows through the front of cowl and over the cylinders properly in order to adequately cool the engine. If the air does not flow over the cylinders as designed they will eventually over-temp, which will lead to cracked heads, burnt exhaust valves, and an increased risk of catastrophic in-flight failure!

Care must be taken to keep the cylinders cool at all times, which is why I installed a JPI EDM-700 engine scanner right after getting 36G. The original factory temp probe is a joke. It is only connected to one cylinder (#4 in 36G's case) and not very accurate. The JPI is connected to each cylinder and records into memory what was going on with the CHT and EGT whenever the unit is powered up. Even if the pilot does not notice the high temps, the EDM-700 will. The data is easy to download to a PC that can be analyzed later. As far as I'm concerned, no high-performance airplane should be flying without one of these -- especially a Turbo!

I was noticing on the last flight that the #1 cylinder was running hotter than the rest and I had to open the cowl flap a bit to keep the CHT temp below 400 degrees. The outside temperature was not outrageous, so this was concerning to me. When you see CHT temps climbing in normal cruse, you need to pay attention to it.

It didn't take long to identify the root cause of the heat problem...

As you can see in the photo, the metal baffle seals are solid without cracks, but the black rubber is deteriorating. Properly functioning baffles need to form a tight seal against the cowling in order to force air to flow over the cylinders. If the baffling is not sealed properly, the air will take the path of least resistance and flow through the openings instead of around the cylinders. Inadequate baffling will also increase drag by allowing more air to flow into the engine compartment than needed.

Baffles are often overlooked. They are on every air cooled airplane, but you generally can't even see them until after the cowl is removed. Typically most owners never remove the entire cowl -- except for annual engine inspection. When the cowling is disassembled, mechanics and owners alike often focus more on doing whatever it is that motivated them to open the clamshell instead of looking at the baffles -- this is especially true of a Mooney since it can take an hour plus to strip a cowling. The end result is that there are many airplanes flying with inadequate cooling to some, or all, of their cylinders.

How to Test Baffles
The only effective test is to check the seals while the cowl is on the plane. It isn't easy to test airflow, but most everyone has a portable light. What I do is put a bright light into the bottom of the cowl (usually through an exhaust opening) and look through the front opening (where the air comes in) in a dark hangar. Properly installed baffles should entirely block the light around the edges of the seals. If you can see any light, they are not sealed properly.

36G failed the light test, so going in I knew I was going to have to replace the seals. I also knew that like many other near 20-year old rubber parts, they were simply past their lifetime and needed replaced. The decision was clear, I had to replace the baffles.

What Baffles Should I Use?
Baffles are generally custom fitted to the cowl and they are one of those strange category of parts that are on the engine, but considered an airframe not a powerplant component. Mooney doesn't sell them since they custom made them during assembly, Continental doesn't sell them since they are not an engine part, you can't find used ones, etc.

In most cases, a mechanic will buy a roll of baffle seal and trim it to fit. I don't like the generic rolled baffle seal because it deteriorates after a few years. It is also very difficult to cut the seals and come out with a good fit and finish. Most replacement baffles look like they were made in a garage. This is unacceptable for 36G's extreme makeover. I only want the very best. There had to be another option...

I noticed at Oshkosh this year that the Lanceair, Mooney and Cirrus use a top of the line rubber seal that has fabric woven into it. The big engines they mount on those guys need to stay cool, so they use a very efficient cooling system that is designed to handle the high heat and stress of the big boar engine. This material is perfect for 36G; however, the little 210 HP TSIO-360 does not have the same cowl configuration as a Cirrus! My goal was to find a supplier that sells the material so I can customize it for 36G.

It took some poking around, but I found it! A company called Sacramento Sky Ranch sells the material by the sheet. You can buy it, use the old seals as a template and cut away! This was a good find; however, as I was looking around I found a company called Gee-Bee Aero Products that is run by a Guy Ginbey. They are a small shop, but what they do is take the raw material and laser cut it to specs. Guy has traced a ton of engine baffles and has the templates already in a computer. This is a perfect fit for our 36G makeover.

Guy at Gee Bee is a super nice fellow, but they don't have a website. You have to call him or email. They can be reached at 800-556-3160 or via email at Guy isn't always around to answer the phone, but he is generally very responsive if you leave him a voicemail or send an email.

Installing Baffles
The first step in the process is to remove the old baffles. This can often be easier said than done. In 36G's case, the baffles were stapled onto the metal heat shields. The key here is to slowly and carefully remove the staples. Start working from the back to open them up and then work them out from the font.

An alternative to prying staples out with a screwdriver is to cut them with a heavy duty wire cutter or tin snip. I don't like doing it this way because it is very easy for one of the small pieces to get somewhere it shouldn't. If care is taken, however, cutting the staples saves a ton of time.

Care should be taken not to destroy the old baffles. You may need to use them as a template. I also mark them in 1, 2, 3, etc. order so I know where they were mounted. This makes the task of laying out the new baffles a breeze.

After removing the old staples and baffle material, the area should be throughly cleaned and checked for cracks. Any cracks should be stop drilled, welded or the piece replaced.

Then the new baffles need to be laid out to make sure that they will fit properly...

After verifying that all will fit, it is time to install them. You cannot be too careful here. This is a measure 10 times drill once ordeal. When the baffles start going on there is zero room for error. You can't make them longer and they are custom cut to fit with no spare parts.

Mounting the seals is an easy process. You simply secure them with pop rivets and metal washers. Care must be taken at this step to not drill into something you shouldn't!

After they are all mounted, the next step to to seal the joints with a high-temperature RTV sealant. Any locations not sealed correctly will leak, so I use a good RTV and clamps to hold the pieces in place until they dry.

The final product is significantly better than the original factory seals. It is easy to care for, durable and will likely last for another 20+ years.

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